An estimated 170 million people worldwide are reported to be infected with hepatitis C virus (HCV), the causative agent of hepatitis C. Seventy to eighty percent of HCV infections lead to chronic liver infection, which in turn may result in severe liver disease, including liver fibrosis, cirrhosis, and hepatocellular carcinoma (115).
HCV constitutes the Hepacivirus genus of the family Flaviviridae (106), and contains a positive-stranded 9.6 kb RNA genome. The features of the HCV genome include a 5′-untranslated region (UTR) that encodes an internal ribosome entry site (IRES) that directs the translation of a single long open reading frame (ORF) encoding a polyprotein of 3,010 amino acids. The HCV ORF is followed by a 3′-UTR of variable length, depending on the HCV variant, that encodes the sequences required for the initiation of antigenomic strand synthesis (79).
The HCV IRES and 3′-UTR both encode regions of RNA structures that are required for genome translation and replication. The HCV polyprotein is posttranslationally processed into at least 10 mature viral proteins, including the structural proteins core (putative nucleocapsid), E1 and E2 and the nonstructural (NS) proteins NS2 to NS5B.
Three distinct elements have been shown to be involved in HCV IRES-mediated translation: (1) integrity of the global structure of HCV IRES, (2) the 3′-terminal region of the HCV genome; and (3) trans-acting cellular factors that interact with the HCV IRES element and assist in translation initiation (35).
The initiation of protein synthesis in eukaryotic cells predominantly follows the 5′ cap-dependent, first AUG rule (61). However, an increasing number of viral (6, 12, 28, 31a, 50, 95, 97, 98, 105, 128) and cellular mRNAs (18, 39, 45, 78, 91, 130) have been shown to use an IRES element to direct translation initiation. In 1992, an IRES element was reported in the 5′ UTR of the HCV RNA genome (129), indicating that synthesis of the viral protein is initiated in a cap-independent fashion.
A bicistronic expression system can be used to define and evaluate the function of IRES elements. This test system harbors two different reporter genes in which the 5′-proximal reporter gene is expressed by a cap dependent translation mechanism while the second reporter is expressed only if an upstream sequence inserted in the intergenic space contains an IRES sequence element. Using this system, a putative IRES in the HCV 5′ UTR was unambiguously demonstrated to function as an IRES involved in translational control of viral proteins (133). In vitro translation, RNA transfection, and mutagenesis studies provided further evidence that the HCV 5′ UTR contains an IRES element (23, 41, 42, 108, 129, 132, 133, 134). Both in vitro and cell-based studies demonstrated that the HCV IRES guides cellular translation initiation factors to an internal site of the viral RNA (56, 58, 120), thus functionally demonstrating the HCV IRES activity. Taken together, these results demonstrate that the HCV 5′-UTR contains an IRES element that plays an active and crucial role in the mechanism of internal initiation for HCV protein translation.
The IRES is one of the most conserved regions of the HCV genome, reflecting its essential nature for viral replication and protein synthesis (13, 118, 122). Although both 5′ and 3′ sequences of the IRES appear to play a role in the control of initiation of translation (42, 109, 110, 113, 136), the minimal sequence requirement for HCV IRES function has been mapped to a region between nucleotides 44-354 (40).
Biochemical probing and computer modeling indicate that the HCV IRES and its 5′ sequence is folded into a distinct structure that consists of four major domains and a pseudoknot (11, 42, 122). Domain I contains a small stem-loop structure that does not appear to be a functional part of the IRES element while domains II, III, and IV contain the HCV IRES activity (43, 111). The relationships between secondary and tertiary structures of the HCV IRES and their function have recently been established (5, 55, 56, 99, 124). Both domains II and III consist of multiple stems, loops, and bulges and are important for IRES activity (23, 40, 51, 52, 54, 56, 64, 74, 75, 93, 107, 108, 110, 124, 127, 131, 139, 141, 142). Domain II can induce conformational changes on the ribosome that have been implicated in the decoding process (124). Domain III has the highest degree of structural conservation among the different HCV strains. It comprises the core of the flavivirus IRES and has 6 subdomains (40). Various studies have shown that subdomain IIId forms complex secondary/tertiary structures and is critical for initiation activity (55, 56, 57, 124, 129). Domain IV has one stem-loop that spans the initiation codon and is specific for the HCV IRES (41, 122), but the precise role of domain IV in IRES activity remains controversial (41, 112).
The role of the HCV IRES is to position the translational machinery near an internal initiator codon in the viral mRNA. The translation initiation mechanism of the HCV and other viral IRES differs significantly from that of 5′-cap-dependent translation initiation (7, 21, 31, 35, 61, 71, 72, 81, 88, 96, 114, 123). Most cellular capped mRNAs utilize a number of initiation factors (eIFs) that are required for the translation initiation process. The initial steps of the process require proteins that interact with the 5′ cap structure and recruit the 40S ribosomal subunit to the cap-proximal region of mRNA. This complex then scans 3′ of the cap, until reaching an AUG codon at which translation will initiate (21, 114). However, in the case of HCV, the IRES functionally replaces the 5′ cap structure, allowing the 40S ribosomal subunit and eIF3 to bind directly to the RNA. Subdomain IIId of the HCV IRES harbors the binding site for the 40S ribosomal subunit and the only initiation factors required for translation initiation are eIF2, eIF3, and eIF4E (15, 58, 94, 100, 120, 124).
The polypyrimidine track-binding protein (PTB) and La autoantigen are noncanonical translation initiation factors that bind to and enhance HCV IRES activity (1, 2, 3, 4, 5, 30, 48, 49, 53). PTB, a 57-kDa protein involved in RNA splicing, is also necessary for efficient IRES-mediated translation initiation of picornavirus mRNA, and some cellular mRNAs (10, 11, 36, 53, 59, 89, 92). The La autoantigen, a 52 kDa double-stranded RNA unwinding protein, also increases the activity of poliovirus and cellular IRES (38, 85, 86). Other cellular factors involved in HCV IRES-mediated translation initiation include proteasome α-subunit PSMA7 (62), ribosomal protein S5 (26), ribosomal protein S9 (24, 25, 100), and hnRNPL (33). However, the role of these RNA-binding proteins in HCV IRES-mediated initiation of translation is unclear. Recently, it was reported that the activity of interferon (IFN) α against HCV replication might target HCV IRES-mediated translation initiation by causing a reduction of La protein levels (117) Some HCV proteins, such as NS5A, core and NS4A/4B, also reported to be involved in the HCV IRES function (143-146). Thus, an inhibitor that blocks interaction between the IRES and the noncanonical factors might efficiently inhibit HCV replication and lack cytotoxicity.
Currently, only interferon (IFN) α and the nucleoside analogue ribavirin, in combination, are marketed for the treatment of HCV infection. However, these two agents are immunomodulators and have limited efficacy, relatively high toxicity, and high cost (80, 83, 84, 138). Although the treatment outcome is variable among the six major HCV genotypes, only about one-half of all treated patients respond to therapy, suggesting that the virus encodes protein products that may directly or indirectly attenuate the antiviral action of IFN. IFNs are naturally produced in response to virus infection, and cellular exposure to IFN leads to the induced expression of a variety of IFN-stimulated genes (ISGs), many of which have an antiviral function. ISG action can limit virus replication at multiple points within the replicative cycle.
There remains a need for an alternative means of treating patients afflicted with HCV. Specifically, a need exists for novel antiviral drugs, for example, that have no cross-resistance with existing treatment modalities, and which demonstrate synergy with other anti-HCV agents.
All documents referred to herein are incorporated by reference into the present application as though fully set forth herein.